1 Introduction

The variability of VZ Cnc (HD 73857) was discovered by Whitney (1950). Since then this star has been referred to as a RR Lyrae or a Cluster-Type variable (Joy & Wilson 1950; Guman 1955; Fitch 1955), as a supershort-period Cepheid (Berdnikov 1975) and as dwarf Cepheid (e.g., Percy et al. 1980) or $\delta$ Scuti-type variable (Cox et al. 1984; López de Coca et al. 1990). Ferro et al. (1994) refer to VZ Cnc as a large amplitude $\delta$ Scuti star. Fitch (1955) and Cox et al. (1984) identify two periodicities in the light variations of VZ Cnc: 0.17836376 and 0.1428041 days as the first and second overtone respectively. A beat period of 0.716292 days is also indicated.

With the observed times of maximum light and the "$\rm O-C$'' technique, many authors have studied the period variations of VZ Cnc (Todoran 1976; Percy et al. 1980; Jiang & Yang 1982; Cao & Jiang 1991; Ferro et al. 1994). Four different interpretations are suggested by researchers as: a sloping linear fit, a sinusoidal fit, abrupt period changes, and long-term effects of nonlinear combinations of short frequencies. However, due to the large scatter in the ($\rm O-C$) residuals, it is difficult to favor a case although the latter two possibilities are regarded to be unrealistic by Ferro et al. (1994).