Long-term variations in the rotational period and O-C diagram of V901 Ori. Left: variation in the rotation period ΔP in seconds with respect to the mean period P₀ = 1ḍ538756(3), derived from 2611 individual measurements. The 1-σ deflections from the quadratic fit are denoted by dashed lines. Points with error bars correspond to virtual period deflections for appropriately selected groups of consecutive individual data items. Right: the difference of the observed zero phase time and the time calculated according to the linear ephemeris in fractions of a day. Each point represents an average of 435 consecutive measurements; the weights of these means are indicated by their areas.