Abstract The ASM/RXTE X-ray light curves of two ultra-compact atoll binaries, 4U 1820–30 and 4U 1915–05, display complicated large-amplitude variations, specific for each system. The ways in which the variations of the mass accretion rate onto the neutron star occur therefore must be largely different in each system. The X-ray light curve of 4U 1820–30, folded with the 172 day cycle, shows the largest scatter on the rising branch and in the dip after the intensity maximum. Episodes of brief low states are superimposed on the general course of the 172 day cycle. The activity can be explained by a hybrid model that combines the mechanism [A&A 77 (1979) 145] and the irradiation-driven instability of the donor star. The previously reported cycle of about 200 days is not detected in 4U 1915–05. Instead, the X-ray light curve displays complicated variations (mostly outbursts, each lasting for about 30 days). We offer an interpretation in terms of the thermal instability of the accretion disk, modified by the irradiation of the disk by the neutron star, incorporating the model [MNRAS 293 (1998) L42].