(14) |

the residuals of these timings are computed. These valuse are listed in the fourth and ninth columns of Table 5 and are plotted in Fig. 6. One photographic minimum 2445057.522 shows large deviation from the general O-C trend formed by other points in Fig. 6. This point was not considered further in the discussion of the period variation.

As displayed in Fig. 6, we can conclude that the orbital period of the system is variable, and its variation is complex. Since the general trend of the diagram may show a roughly parabolic distribution indicating a secular increase in the period, a least-squares solution of the values yields the following ephemeris:

(15) |

where the coefficient of the square term represents the rate of change of the period. This ephemeris can be used for the estimation of future times of minima. A continuous period increase of days/cycle = days/year is calculated which is equivalent to a period increase of 6.1 s/century. As in the case of RX Hya, the secular increase only indicates the general trend of the diagram without describing any particular characteristic.

Figure 6:
The O-C diagram of AC Tau. Also given in solid line
is its description by a quadratic ephemeris |

The residuals from the quadratic ephemeris (15) are also listed in Table 5 and displayed in Fig. 7. The values in Fig. 7 clearly suggest a periodic oscillation. With the least-squares method, the following periodic ephemeris is obtained:

(16) |

which can describe the general trend of the
values wery well (solid line in Fig. 7). This
ephemeris tell us a cyclic variation with a period
of about *T*=29.8year and an amplitude of about
.
The cyclic variation of the O-C curve
indicates that the orbital period varies in a periodic
way.

Figure 7:
Residuals of AC Tau from the quadratic ephemeris
and their description by a periodic function |

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