The Hubble Space Telescope survey of BL Lacertae objects: Gravitational lens candidates and other unusual sources

We present Hubble Space Telescope (HST) observations of seven unusual objec ts from the HST "snapshot survey" of BL Lacertae objects, of which four are gravitational lens candidates. In three cases a double point source is obs erved: 0033 + 595, with 1." 58 separation, and 0502 + 675 and 1440 + 122, e ach with similar to 0." 13 separation. The last two also show one or more g alaxies, which could be either host or lensing galaxies. If any are confirm ed as lenses, these BL Lac objects are excellent candidates for measuring H -0 via gravitational time delay because of their characteristic rapid, high -amplitude variability. An additional advantage is that, like other blazars , they are likely superluminal radio sources, in which case the source plan e is mapped out over a period of years, providing strong additional constra ints on the lensing mass distribution. The fourth gravitational lens candid ate is 1517 + 656, which is surrounded by three arclets forming an almost p erfect ring of radius 2." 4. If this is indeed an Einstein ring, it is most likely a background source gravitationally lensed by the BL Lac object hos t galaxy and possibly a surrounding group or cluster. In the extreme case t hat all four candidates are true lenses, the derived frequency of gravitati onal lensing in this BL Lac sample would be an order of magnitude higher th an in comparable quasar samples. We also report on three other remarkable B L Lac objects: 0138-037, which is surrounded by a large number of close com panion galaxies; 0806 + 524, whose host galaxy contains an uncommon arclike structure; and 1959 + 650, which is hosted by a gas-rich elliptical galaxy with a prominent dust lane of similar to 5 x 10(5) M..