YZ Phoenicis: a very short period K-type contact binary with variation of the O’Connell effect and orbital period change

Abstract

YZ Phe is a very short-period contact binary (Sp. = K2 V) with an orbital period of 0.2347 d near the short period limit (0.22 d). We present the complete light curves which photometric data were obtained from the 60 cm telescope of PROMPT-8 at CTIO in Chile during 2016 June to October and 2017 August. The photometric solutions were determined by using the Wilson & Devinney code and the results reveal that YZ Phe is a W-subtype shallow contact binary ($f\sim 10\,$, q = 2.635, or 1/q = 0.379 for W subtype) with rotational motion of a large hot spot on the more massive component, showing a strong O’Connell effect with variation of maxima in photometric time series at period of 4.20 yr and stellar cycle at period of 1.28 yr. By compiling all available eclipse times, the result shows a long-term period decrease at a rate of dP/dt = −2.64(±0.02) × 10−8 d yr−1, superimposed on a cyclic variation (A3 = 0.0081 d and P3 = 40.76 yr). This variation cannot be explained by the Applegate mechanism. Thus, the cyclic change may be interpreted as the light-travel time effect via the presence of a cool third body. Based on photometric solutions, the third light was detected as $2\,$ of the total light in V and I bands. These results support the existence of a third body. The long-term period decrease can be explained by mass transfer from the more massive component ($M_2 \sim 0.74\, M_{\odot }$) to the less massive one ($M_1 \sim 0.28\, M_{\odot }$) or plus angular momentum loss (AML) via magnetic braking. With 1/q < 0.4 and long-term period decrease, all factors suggest that YZ Phe is on the AML-controlled state and its fill-out factor will increase, as well as the system evolving into a deeper normal contact binary.