Physical Information: Based on the distance estimate of about 320 to 325 million light-years discussed in the entry for PGC 27509 (HCG 40A), PGC 27508's apparent size of about 1.3 by 0.2 arcmin (from the images below) implies that the galaxy is about 120 to 125 thousand light-years across.
Classification Note: The "sp" stands for "spindle", or an edge-on galaxy, which makes determining the type difficult (hence the colon after "ab"). All of the galaxies in the group are affected by their interaction, but PGC 27508 looks more normal than most of the others, so although most of them are classified as "peculiar" in one reference or another, adding a question mark to that for this galaxy seems appropriate.
Use By The Arp Atlas: HCG 40 is used by the Arp Atlas of Peculiar Galaxies as an example of a group of galaxies, with the note "Sharp absorption lane in connection to southern most galaxy." (There is also a note about the position being taken from the VV (Vorontsov-Velyaminov) Catalogue of Interacting Galaxies, but the positions shown on this page are taken directly from modern image databases.)

Physical Information: Since all five members of HCG 40 are within a region only about twice the size of our galaxy (see note below) and physically interacting, they must be at the same distance from us. This means that the best estimate of that distance involves an average of all the information available for them, the gory details of which are shown here and only the result copied to the other four entries.
 For PGC 27509, the recessional velocity relative to the Cosmic Microwave Background (3K Vr) is 6965 km/sec, and redshift-independent distance estimates range from about 255 to 325 million light-years, with a median of about 305 to 310 million light-years. For PGC 27508, the recessional velocity is 6745 km/sec, for PGC 27513 it is 7160 km/sec, for PGC 27515 it is 6975 km/sec, and for PGC 27516 it is 6835 km/sec. This yields an average recessional velocity of about 6935 km/sec, which corresponds to a Hubble Flow distance of about 320 to 325 million light-years, in good agreement with the redshift-independent distance estimates. I have therefore used the 320 to 325 million light-year distance as the distance to HCG 40 for all five members of the Group. Given its apparent size of about 1.15 by 0.9 arcmin (from the images below), that implies that PGC 27509 is about 105 to 110 thousand light-years across.
Comparison of HCG 40 with the Milky Way: As stated above, HCG 40 is only about twice the size of our Milky Way Galaxy. This is based on its apparent size of about 2.4 by 1.5 arcmin (from the images below) and the distance derived above; that combination implies an overall size for the Group of about 225 thousand light-years, and since estimates of the "visible" size of our galaxy (if seen from the outside) range from about 100 to 200 thousand light-years (mostly closer to the lower end of the scale), HCG 40 is certainly no more than twice the size of our galaxy, and only about the same size as our larger neighbor, the Andromeda Galaxy, which is about 220 thousand light-years in diameter.
Use By The Arp Atlas: HCG 40 is used by the Arp Atlas of Peculiar Galaxies as an example of a group of galaxies, with the note "Sharp absorption lane in connection to southern most galaxy." (There is also a note about the position being taken from the VV (Vorontsov-Velyaminov) Catalogue of Interacting Galaxies, but the positions shown on this page are taken directly from modern image databases.)

Physical Information: Based on the distance estimate of about 320 to 325 million light-years discussed in the entry for PGC 27509 (HCG 40A), PGC 27513's apparent size of about 0.8 by 0.45 arcmin for its outer ring and about 0.3 by 0.2 arcmin for its bright center (from the images below) implies that the central galaxy is about 25 to 30 thousand light-years across, while the outer ring spans about 75 thousand light-years.
Use By The Arp Atlas: HCG 40 is used by the Arp Atlas of Peculiar Galaxies as an example of a group of galaxies, with the note "Sharp absorption lane in connection to southern most galaxy." (There is also a note about the position being taken from the VV (Vorontsov-Velyaminov) Catalogue of Interacting Galaxies, but the positions shown on this page are taken directly from modern image databases.)

Physical Information: Based on the distance estimate of about 320 to 325 million light-years discussed in the entry for PGC 27509 (HCG 40A), PGC 27515's apparent size of about 0.6 by 0.15 arcmin (from the images below) implies that the galaxy is about 55 to 60 thousand light-years across.
Classification Note: As indicated by "sp", PGC 27515 is a "spindle" and nearly edge-on, which makes determining its type difficult; hence the colons in the type shown above. "pec" is most obvious in the twisted ends of its disk, but if the Group was closer there would almost certainly be other evidence of the chaos created by its interaction with its (very near) neighbors.
Use By The Arp Atlas: HCG 40 is used by the Arp Atlas of Peculiar Galaxies as an example of a group of galaxies, with the note "Sharp absorption lane in connection to southern most galaxy." (There is also a note about the position being taken from the VV (Vorontsov-Velyaminov) Catalogue of Interacting Galaxies, but the positions shown on this page are taken directly from modern image databases.)

Physical Information: Based on the distance estimate of about 320 to 325 million light-years discussed in the entry for PGC 27509 (HCG 40A), PGC 27516's apparent size of about 0.65 by 0.35 arcmin (from the images below) implies that the galaxy is about 60 to 65 thousand light-years across.
Apparent size of about 0.65 by 0.35 arcmin (from the images below). Its outer ring has extensive dust lanes as well as a faint background of presumably innumerable stars, and is aligned more or less east-west, whereas the central galaxy is tilted upward on the east and downward on the west by about 30 degrees, which would be enough to classify this as a peculiar galaxy even without other signs of distortion as a result of gravitational interaction with its (very close) neighbors (although HCG 40 contains five galaxies, it spans only about twice the space occupied by our own galaxy, so the gravitational effects of the various galaxies on each other are very obvious).
Use By The Arp Atlas: HCG 40 is used by the Arp Atlas of Peculiar Galaxies as an example of a group of galaxies, with the note "Sharp absorption lane in connection to southern most galaxy." (There is also a note about the position being taken from the VV (Vorontsov-Velyaminov) Catalogue of Interacting Galaxies, but the positions shown on this page are taken directly from modern image databases.)

Misidentification as a Galaxy: In older images (see PGC 27508 for a DSS image of the region), the star superimposed on the southeastern side of PGC 27516 slightly resembles a compact galaxy, and as a result it was listed by Hickson as a member of HCG 40; but better images show that it is only a star in our own galaxy, and should therefore be removed from the Group.
Additional Error Notes: A search of HyperLEDA for HCG 40F brings up a page for PGC 27517, which still lists it as an elliptical galaxy. SIMBAD makes the same error, and even lists a recessional velocity of more than 6200 km/sec, which is an obvious blunder. Those errors mean that this entry needs to be here, as a warning about the mistake. NED lists the star as PGC 27517, but does not mention its former membership in HCG 40, and although there is no mention of the fact that it is only a star, the page only lists it as a source of infrared and ultraviolet radiation, which as a reasonably hot star (based on its color), it should be.

Physical Information: (Note: NED and LEDA agree that the eastern member of the pair is PGC 27546, and the western member is PGC 27547; but LEDA has the galaxy types reversed, and NED lists each galaxy as lenticular in one place and spiral in another place. So there has undoubtedly been considerable confusion about the correct identification of the galaxies, and whether their physical data are correctly given in the two catalogs is questionable. As a result, the brightness, apparent size and galaxy types given here are based directly on the images below; but which galaxy has a given recessional velocity is anybody's guess.) PGC 27546 is generally considered a member of an interacting pair (with PGC 27547), in which case their individual recessional velocities of 6860 and 6585 km/sec should be averaged to determine their distance (and since there is a very good chance that those velocities have been misattributed, it might be best to use their average anyway). Based on their average recessional velocity of 6820 km/sec, PGC 27546 and PGC 27547 are about 315 million light years away. Given that and its apparent size of 0.5 by 0.45 arcmin, PGC 27546 is about 45 thousand light years across. It is used by the Arp Atlas as an example of an elliptical galaxy (PGC 27546) close to and perturbing a spiral galaxy (PGC 27547).

Physical Information: (Note: NED and LEDA agree that the eastern member of the pair is PGC 27546, and the western member is PGC 27547; but LEDA has the galaxy types reversed, and NED lists each galaxy as lenticular in one place and spiral in another place. So there has undoubtedly been considerable confusion about the correct identification of the galaxies, and whether their physical data are correctly given in the two catalogs is questionable. As a result, the brightness, apparent size and galaxy types given here are based directly on the images above; but which galaxy has a given recessional velocity is anybody's guess.) PGC 27547 is generally considered a member of an interacting pair (with PGC 27546, which see for images), in which case their individual recessional velocities of 6860 and 6585 km/sec should be averaged to determine their distance (and since there is a very good chance that those velocities have been misattributed, it might be best to use their average anyway). Based on their average recessional velocity of 6820 km/sec, PGC 27546 and PGC 27547 are about 315 million light years away. Given that and its apparent size of 0.7 by 0.55 arcmin, PGC 27547 is about 65 thousand light years across. It is used by the Arp Atlas as an example of an elliptical galaxy (PGC 27546) close to and perturbing a spiral galaxy (PGC 27547).