Period changes in four algol-type binaries: SX Dra, AV Del, CU Peg and DK Peg

Orbital period changes of four neglected short-period semidetached Algols, SX Dra, AV Del, CU Peg and DK Peg, are presented based on the analysis of their O-C observations. It is found that the three systems, AV Del, CU Peg and DK Peg, show rapid period increase at rates of dP/dt = +3.15 × 10-6, +2.00 × 10-6 and +4.88 × 10-7 days/year respectively. For the other system, SX Dra, a possible periodic variation in the orbital period is discovered to superimpose on a very rapid period increase (dP/dt - +1.09 × 10-5 days/year). The period increases of the four Algols can be explained by a mass transfer from the secondary to the more massive primary, which is in agreement with the semi-detached configurations of these systems. However, since the period increases are rather rapid, the mass transfer should be on a dynamical time scale. The cyclic period change in SX Dra suggests that there may exist a third body in this system.