PHOTOMETRIC PROPERTIES OF SELECTED ALGOL-TYPE BINARIES. III. AL GEMINORUM AND BM MONOCEROTIS WITH POSSIBLE LIGHT-TIME ORBITS

Abstract

We present the CCD photometry of two Algol-type binaries, AL Gem and BM Mon, observed from 2008 November to 2011 January. With the updated Wilson–Devinney program, photometric solutions were deduced from their EA-type light curves. The mass ratios and fill-out factors of the primaries are found to be qph = 0.090(± 0.005) and f1 = 47.3%(± 0.3%) for AL Gem, and qph = 0.275(± 0.007) and f1 = 55.4%(± 0.5%) for BM Mon, respectively. By analyzing the O−C curves, we discovered that the periods of AL Gem and BM Mon change in a quasi-sinusoidal mode, which may possibly result from the light-time effect via the presence of a third body. Periods, amplitudes, and eccentricities of light-time orbits are 78.83(± 1.17) yr, 00204(±00007), and 0.28(± 0.02) for AL Gem and 97.78(± 2.67) yr, 00175(±00006), and 0.29(± 0.02) for BM Mon, respectively. Assumed to be in a coplanar orbit with the binary, the masses of the third bodies would be 0.29 M☉ for AL Gem and 0.26 M☉ for BM Mon. This kind of additional companion can extract angular momentum from the close binary orbit, and such processes may play an important role in multiple star evolution.