Detection of a cool stellar companion to the δδ Scuti variable star BP Peg

• The pulsating period of BP Peg shows a cyclic oscillation.
• Its cyclic oscillation with a period of 24.5 years and an amplitude of 0.0060 days.
• BP Peg is a binary star system, the hidden companion mass could be 0.20 M⊙M⊙.

New CCD photometric monitoring on the high-amplitude δδ Scuti variable BP Peg was carried out on November 4, 2013 by using the 1.0-m reflecting telescope of Yunnan Observatories in China as well as on December 29, 2013 by using the 85-cm telescope at Xinglong Station of National Astronomical Observatory in China. BP Peg is a new high-amplitude δδ Scuti variable with a period of 0.10954357 day. We have collected all the times of light maximum from the literature together with our data, and the pulsation period changes are analyzed through Observed–Calculated (O–C)(O–C) method. With the available data, we discover that the pulsating period of BP Peg shows a cyclic oscillation with a period of 24.5 years and an amplitude of 0.0060 days. If the cyclic period changes are caused by light-time effect in a binary system, the reckoned mass of the hidden companion could be 0.20 M⊙M⊙ and possibly, it is an M-type star.