  Recent theoretical and observational studies have shown that ashes from
thermonuclear burning may be ejected during radius-expansion bursts, giving
rise to photoionisation edges in the X-ray spectra. We report a search for such
features in Chandra spectra observed from the low-mass X-ray binary 4U 1728-34.
We analysed the spectra from four radius-expansion bursts detected in 2006
July, and two in 2002 March, but found no evidence for discrete features. We
estimate upper limits for the equivalent widths of edges of a few hundred eV,
which for the moderate temperatures observed during the bursts, are comparable
with the predictions. During the 2006 July observation 4U 1728-34 exhibited
weak, unusually frequent bursts (separated by <2 hr in some cases), with
profiles and alpha-values characteristic of hydrogen-poor fuel. Recurrence
times as short as those measured are insufficient to exhaust the accreted
hydrogen at solar composition, suggesting that the source accretes hydrogen
deficient fuel, for example from an evolved donor. The detection for the first
time of a 10.77 min periodic signal in the persistent intensity, perhaps
arising from orbital modulation, supports this explanation, and suggests that
this system is an ultracompact binary similar to 4U 1820-30.
