Photospheric expansion has been observed during several powerful type I X-ray bursts, and it is associated with a supersonic outflow driven by the super-Eddington energy release in the thermonuclear He-burning at the base of the envelope (Lewin et al. 1993). Such expanded atmospheres represent perfect background for the propagation of sound, and we believe there is a number of arguments in favor of the reality of these phenomena.

We have suggested an acoustic origin for the second range photospheric oscillations detected in the X-ray bursters 1608-522 and 2127+119 (Lapidus et al. 1994). On the basis of our models of radiation driven winds from neutron stars during strongest bursts (Nobili et al. 1994), we have shown that purely radial standing sound waves in expanded spherical envelopes can have periods very close to the observed ones. Both the photosphere and the sonic surface may act as a reflecting boundary; this is the reason of quite different periods in the two sources (0.65 s and ∼ 7 s, respectively).