The variation of the period of RZ Cassiopeiae.

A paper by A. de Sitter published in 1933 in the Bulletsn of the A stronomical Inst~tutes of the Netherlands represented the earlier observations of the times of minima of RZ Cassiopeiae by an average period with variations having a period of 5499 epochs and amplitude 0.005 day. Since de Sitter's formula does not satisfy the photoelectric observations made at the Washburn Observatory during the interval I94~48, an attempt has been made to improve the formula. The~following well-determined times of heliocentric minima observed photoelectrically have been added to those used by de Sitter: Helioceotric J.I~.12 1~iiase 0-C 2429875.6902 10475 +.0042 .0000 2430028.6812 10603 +.0031 -.0010 0322.7125 10849 +.0026 - .0010 2085.7011 12324 - .0040 - .0002 2115.5827 12349 - .0038 .0000 2152.6358 12380 - .0035 +.00OI 2452.6460 12631 - .0019 +.0004 2820.7853 12939 - .0005 - .0004 2862.6189 12974 - .ooo6 - .0009 The epochs are numbered as in Dugan's and de Sitter's papers. The phases in the table were computed by the formula: Phase = 0.836643729 (J.D. -2432862.6197). The variation is represented by 0d0041 sin (E - 195) 336900 The representation of the minima published in de Sitter's paper is very poor. Both Dugan and de Sitter used normal points with very large scatter. The Washburn Observatory minima are determined from observations made on single nights, the last three from measures recorded on an Esterline-Angus recording meter. Because of the poorly determined early minima, the preliminary period of the variation-3I90 epochs is very uncertain. If accurate observations of the times of minima between 1933 and 1940 are available, it should be possible to decide between the two periods, 5499 E and 3190 E. Washburn Observatory, Madison, Wis.