The calculations with the use of simple maser model show that the stellar background contributes negligibly to the mean peak intensities at 1612 MHz. Under some conditions, however, the contribution from the central star may constitute a few percent of the mean peak intensity. Such possibility may occur when the thick and large OH envelope is illuminated by very luminous and extended central star. To compare the theoretical results with observations a sample of 55 OH/IR stars with best known parameters is analyzed. The ratio of the blue-shifted to the red-shifted 1612 MHz peak slightly increases in objects with the largest OH radii. This relation is interpreted in terms of amplification of the stellar photons.