Time variations of orbital period (solid lines, right axis) and component masses (broken lines, left axis) of the best fitting model of V209 (see Table 2 for its initial parameters). Also shown are two models with the same initial component masses, and periods differing by ±0.02 d from the best-fitting one. The observed parameters of V209 are indicated by a filled circle (period) and diamonds (masses). Vertical sizes of the diamonds correspond to observational errors.