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Description
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 f_1_=47.3%(+/-0.3%) for AL Gem, and q_ph_=0.275(+/-0.007) and f_1_=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 77.83(+/-1.17)yr, 0.0204(+/-0.0007)d, and 0.28(+/-0.02) for AL Gem and 97.78(+/-2.67)yr, 0.0175(+/-0.0006)d, 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 0.29M_{sun}_ for AL Gem and 0.26M_{sun}_ 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.
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