An RR Lyrae Star Survey with Ihe Lick 20-INCH Astrograph II. The Calculation of RR Lyrae Periods by Electronic Computer.

A program is described for finding the period of an RR Lyrae variable from magnitude observations by means of an electronic computer such as the IBM 1620. For a number of trial periods in the appropriate range, the computer forms the sum (0) of the squares of the magnitude difference between observations of adjacent phase using a group of the observations which are most concentrated in time. A number (e.g., three) of these periods for which 0 has the smallest values are selected as being the most likely to be correct. For each of these three periods, all the observations are used to compute 0 for periods in decreasing ranges around the period for which 0 had a minimum value in the previous calculation until the required accuracy is achieved. The finally selected period is that for which 0 is a minimum. The ratio (E)) of o to the sum of the squares of the differences of all the magnitudes from their mean value may be used to estimate the reliability of the period obtained. Expressions are given for the number of trial periods needed and the number of observations required as a function of amplitude of the variable and rms error of the observations at different levels of discrimination against spurious periods. The origin and probability of occurrence of spurious periods are briefly reviewed.