5 Chance projections and triple systems

In the vicinity of five programme stars we found an additional object within a radius of 3 $.\!\!{\hbox{$^{\prime\prime}$ }}$0. We have demonstrated above that only in two cases (TON 139 and PG 1718+519) the relative brightnesses are consistent with the expectations from the deconvolution of the spectral energy distribution. The remaining three cases must then be chance projections or triple systems. Since the programme stars lie at high galactic latitudes (except KPD 2215+5037, see Table 1), we expect chance coincidences to be rare. Indeed, we do not find any additional object in the PC field (40 $^{\prime \prime }$$\times$40 $^{\prime \prime }$) except for the low galactic latitude object KPD 2215+5037.

According to Abt & Levy (1976) 16% of multiple systems of normal stars are triples. If the fraction of triple systems is the same for our sample, we expect three programme stars to be triple. Most of these, if not all, should be resolvable. Besides TON 139 and PG 1718+519 we find in three cases companions to the sdB stars which are too faint to match the spectral energy distribution. These could be triple systems consisting of an unresolved sdB binary and a distant third star.