  The symbiotic system HM Sagittae consists of a Mira star and a secondary
White Dwarf component. The dust content of the system was severely affected by
the nova outburst in 1975, which is still ongoing. The capabilities of optical
interferometry operating in the mid-IR allow us to investigate the current
geometry of the dust envelope. We test our previous spectro-interferometric
study of this system with new interferometric configurations, increasing the uv
coverage and allowing us to ascertain the appearance of the source between 8
and 13micron. We used the MIDI instrument of the VLTI with the unit telescopes
(UTs) and auxiliary telescopes (ATs) providing baselines oriented from
PA=42degrees to 127 degrees. The data are interpreted by means of an elliptical
Gaussian model and the spherical radiative transfer code DUSTY. We demonstrate
that the data can be reproduced well by an optically thick dust shell of
amorphous silicate, typical of those encountered around Mira stars, whose
measured dimension increases from 8 to 13micron. We confirm that the envelope
is more extended in a direction perpendicular to the binary axis. The level of
elongation increases with wavelength in contrast to our claim in a previous
study. The wider uv coverage allows us to deepen our previous investigations of
the close circumstellar structure of this object.
