1 Introduction

This paper presents differential Strömgren uvby observations of four magnetic Chemically Peculiar (mCP) stars HD 35298, 19 Lyr, HD 192678, and HR 8216. Studies of the mCP stars using data from the Four College Automated Photoelectric Telescope (FCAPT) have both improved our knowledge of their rotational periods and better defined the shapes of their light curves (see, e.g. Adelman & Brunhouse 1998 and references therein). These results can be used to better relate observations taken at different times and to study the period distribution of mCP stars. Their variable light curves provide information concerning the uniformity of the surface abundances. Hydrodynamical processes including diffusion and gravitational settling in radiative atmospheres with strong magnetic fields most likely produce the anomalous photospheric abundances of the mCP stars. Their abundance distributions are patchy and affect the emergent flux distribution. As their magnetic and rotational axes are not aligned, a distant observer will see photometric, magnetic, and spectrum variability as the stars rotate (Michaud & Proffitt 1993 and references therein).

The FCAPT operated on Mt. Hopkins, AZ between September 1990 and July 1996 and since then on nearby Washington Camp, AZ. After the dark count, the telescope measures the sky-ch-c-v-c-v-c-v-c-ch-sky in each filter where sky is a reading of the sky, ch that of the check star, c that of the comparison star, and v that of the variable star. Table 1 contains group (a variable along with two supposedly non-variable stars, the comparison and check, against which the brightness of the variable is compared) information (Hoffleit 1982; Hoffleit et al. 1993). Corrections were not made for neutral density filter differences among the stars of each group. The comparison and check stars were chosen from supposedly non-variable stars in the vicinity of the variable on the sky that had similar V magnitudes and B-V colors. Adelman (1998) checked their stability using Hipparcos photometry (ESA 1997). We used the Scargle periodogram (Scargle 1982; Horne & Baliunas 1986) and the clean algorithm (Roberts et al. 1987) in finding periods.

  

Table 1: Photometric groups