Abstract
Strömgren four-color photometric measures have been made of blue horizontal-branch A stars in the globular clusters M 4, M 13 and M 55 with the Steward Observatory 90 inch telescope and with the 60 inch telescope at Cerro Tololo Inter-American Observatory. These stars are faint, ranging in V magnitude from 13.6 in M 4 to 15.5 in M 13 and the corresponding errors in the four-color indices are ± 0.04 to 0.06 in the c1 index, for one observation. The error of the mean value of the c1 indices is approximately ± 0.02 for most of the stars since they have been measured from 4 to 10 times each.
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