The globular cluster Messier 10 has three known variables. The first two of these were discovered by one of us (Sawyer 1933) and the third by Arp (1955). Two of the variables, V2 (P=18.7226) and V3 (P=7.831), are population II cepheids while V1 appears to be an irregular variable. Another star which lies in the Schwarzschild gap on the horizontal branch is a suspected variable (Voroshilov 1971).
In this investigation, we examine the variations in the periods of the two cepheids over the interval 1912 to 1983 (for V2) and 1931 to 1983 (for V3). The study is based on photographs obtained with seven different telescopes - the Mt.Wilson 100-inch and 60-inch (1912 to 1919), the Dominion Astrophysical Observatory 72-inch, the David Dunlap 74-inch and 19-inch, the 16-inch at the University of Toronto downtown campus and the University of Toronto 24-inch at the Las Campanas Observatory of the Carnegie Institution of Washington. Some of our magnitudes have already been published (Sawyer 1938) and the remaining ones will be submitted to the Astronomical Journal for publication. We have also included material published by Arp (1955, 1957) in our study.