A cluster of intermediate age in the Large Magellanic Cloud

Some years ago (1960) I discussed the HR diagram of the variable star field investigated at Herstmonceux as the result of Drs. Sandage and Eggen's visit to Pretoria, and found colour-luminosity arrays showing mainly very blue giant stars; and later (1961) I suggested that a second and older population could be seen in the same area of the LMC, showing a rather different x,y distribution. Recently Miss Epps has assisted me to make further investigations into the matter. We are now discussing stars fainter than V = 14 · 5 and therefore too faint for us to determine their proper motions. In order to get rid of foreground stars which obscure the HR diagram we have had to make use of the x, y distribution to discriminate between LMC members and foreground stars. Firstly, we examined an open cluster which appears as a faint object on the 74-inch Pretoria plates. It is away from the main lane occupied by the bright blue objects, and its appearance on the B and V plates showed it not to be excessively blue. On a Radcliffe plate (74 in. stopped to 44 in.) with an exposure of 20 min, 29 stars can be seen within a circle of diameter of 1′ of arc. We have been able to determine V accurately for only 15 of these stars, one of which proved to be a variable of period 4–75 days. The HR diagram shows a gap between B – V = 0 · 2 and B – V = 1 · 0, the variable being in this gap. It is comparable with those of NGC 129, NGC 2287, and NGC 6067, and, by analogy with these objects, the open cluster now described probably has an age of about 108 years. The object is of course very different from the young or blue (but tight) cluster NGC 1818 not far away from it.