Short-term optical variability of HD 153919 = 4U1700-37

Photometric observations of HD 153919 are presented, which indicate optical cyclic variations with an amplitude of up to 0.02 mag, and a strong dependence of the amplitude on the orbital plane. From power spectrum analysis, a value of 99.9 min from the observations of van Paradijs et al. (1978) has been determined for the characteristic period. In addition, evidence suggests a secular decrease of the period with a rate of decrease of -0.03 per year.