1 Introduction

The anomalous photospheric abundances of the magnetic Chemically Peculiar (mCP) stars, whose effective temperatures and surface gravities are similar to those of main sequence B, A, and early F stars, are thought to be produced by hydrodynamical processes including radiative diffusion and gravitational settling in radiative envelopes which have strong magnetic fields (Michaud & Proffitt 1993 and references therein). The resulting abundance distributions depend upon the magnetic field, should be patchy, and affect the emergent flux distribution (Shore & Adelman 1974). This is necessary to explain their photometric, magnetic, and spectrum variability. Differential photometric studies with the 0.75-m Four College Automated Photoelectric Telescope (FCAPT) have both improved periods, which are crucial to relating observations taken at different times, and better defined the shapes of their light curves (see, e.g. Adelman & Brunhouse 1998). These results can be used to detect changes in light curves between observing seasons, deduce information concerning the uniformity of the surface abundances, and study the period distribution of mCP stars. An important goal in this research is to derive abundances as a function of photospheric position and then use these abundances to test the theories of chemical differentiation. This permits investigators to learn about hydrodynamical processes at work in mCP (and other hot) star atmospheres.

  

Table 1: Photometric groups

^aHD 165358 was used as the check star for 1995-97 and HD 166228 for 1997-98.

This paper presents single-channel differential Strömgren photometry of four mCP stars obtained with the FCAPT, now at Washington Camp, AZ after being on Mt. Hopkins, AZ for six years (September 1990 - July 1996). For these stars Strömgren phtometry yields more astrophysical information than UBV photometry. In part this is due to the $\lambda$5200 broad, continuum feature's effect on y values. For each photometric group (Table 1) of variable, comparison, and check stars, the telescope measures the dark count and then the sky-ch-c-v-c-v-c-v-c-ch-sky in each filter where sky, ch, c, and v are respectively readings of the sky, the check star, the comparison star, and the variable star. The comparison and check stars were selected from supposedly non-variable stars near the variable on the sky that had similar V magnitudes and B-V colors using the Bright Star Catalogue (Hoffleit 1982) and the experience of other photometrists and checked later using photometry from the Hipparcos satellite (ESA 1997). Tables 2-5 contain the data and their yearly means and standard deviations. Corrections were not made for neutral density filter differences among the stars of each group. The standard deviations in Tables 2-5 of the check-comparison star differences indicate that these stars are constant for the period when they were observed.

We plot the v-c data for each variable star with the best published period to see if our data approximately confirm this period. Then we use the Scargle periodogram (Scargle 1982; Horne & Baliunas 1986) and/or the clean algorithm (Roberts et al. 1987) with our data. Finally we adjust the period to make our data and any published data coincide as well as possible in phase. In Figs. 1-4 our values are indicated by plus signs, those of Winzer (1974) converted to our zero points as closed circles except for 49 Her where they are closed diamonds, and those of Burke & Barr (1981) by closed diamonds for HR 6718.