COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1325 Konkoly Observatory Budapest 1977 August 12 XX CEPHEI: NEW TIMES OF MINIMUM AND A STUDY OF THE PERIOD The A8 single-spectrum (Struve,1946) eclipsing system XX Cephei, was observed photoelectrically with the 60 cm telescope at Loiano (Bologna). Nine new times of minimum, derived by the Kwee-Van Woerden's method are reported in Table I, along with the standard errors, the number of single observations used in the determination of the minima and the colour of the observations (columns 1,2,3 and 4, respectively). Table I J.D. hel sigma n colour (-2400000) I 40506.4019 .0002 86 blue I 40866.3467 .0004 8 blue I 40866.3485 .0006 8 yellow I 40887.3841 .0007 18 blue I 40887.3841 .0007 19 yellow I 41539.4973 .0003 36 blue I 41539.4972 .0002 39 yellow II 41608.443 .003 30 yellow II 41622.476 .001 44 yellow Several authors pointed out a variability of the orbital period (Fresa, 1953; Lavrov, 1957; Koch and Koch, 1962; Kopal, 1965). The presence of an apsidal motion was suspected by Fresa (1953). In his classical paper Kopal (1965) indicated the possibility of an apsidal motion and, on the basis of Iljasova (1946), Struve (1946), and Fresa's (1953) data he was able to calculate a value U/P=10000 +- 1200. With the present data we can rule out this possibility. Table II collects these data, with no claim of completeness. We underline the fact that any argument founded on the analysis of the secondary minimum must be considered with caution. This minimum is in fact very shallow, about 0.03m in blue and yellow colours, according to our well observed light-curve. The period, however, appears not to be constant: in fact the primary minima cannot be represented by a unique linear relation. For the first sixteen minima (Table II) a least squares solution gives: Min. = JD 2425096.449 +2.337340d E (1) +-. 8 +-. 4 For the remaining ones the least squares linear ephemeris turns out to be: Min.= JD 2441539.4917+ 2.3373059d E (-4801=