CCD ensemble photometry on an inhomogeneous set of exposures

A method is described to perform ensemble stellar photometry on a series of CCD images for which the number and identity of the comparison stars vary throughout the set of exposures. The technique is particularly useful for the production of light curves of variable stars from long-term photometric programs where the inhomogeneity of the data set can be large. The linear least squares solution is derived, paying particular attention to the evaluation of errors. Examples are presented of the use of two interactive computer programs which implement this technique.