2012AJ....143...13Y

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 q_ph = 0.090(± 0.005) and f₁= 47.3%(± 0.3%) for AL Gem, and q_ph= 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, 0.<SUP>d</SUP>0204(±0.<SUP>d</SUP>0007), and 0.28(± 0.02) for AL Gem and 97.78(± 2.67) yr, 0.<SUP>d</SUP>0175(±0.<SUP>d</SUP>0006), 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.