Turn-on history of Her X-1: the (O–C) diagram (using 20.5    ×    P_orb as the period to calculate the expected turn-ons). Upper: the anomalous low around MJD 51 500 (AL3) is assumed to last for eighteen 35 d cycles. Lower: the same assuming that there were only seventeen 35 d cycles (see Staubert et al. 2009). The solid line represents the linear best fit to all data, defining a mean period of 34.98 d (which may be used for a rough ephemeris of turn-ons).