  Most of the known debris discs exhibit cool dust in regions analogous to the
Edgeworth-Kuiper Belt. However, a rare subset show hot excess from within a few
AU, which is often inferred to be transient. We examine 2 such sources to place
limits on their location to help distinguish between different interpretations
for their origin. We use MIDI on the VLTI to observe the debris discs around
eta Corvi and HD69830 using baseline lengths from 44-130m. New VISIR
observations of HD69830 at 18.7um are also presented. These observations are
compared with disc models to place limits on disc size. The visibility
functions measured with MIDI for both sources show significant variation with
wavelength across 8-13um in a manner consistent with the disc flux being well
resolved, notably with a dip at 10-11.5um due to the silicate emission feature.
The average ratio of visibilities measured between 10-11.5um and 8-9um is
0.934+/-0.015 for HD69830 and 0.880+/-0.013 for eta Corvi over the 4 baselines
for each source, a departure of 4 and 9sigma from that expected if the discs
were unresolved. HD69830 is unresolved by VISIR at 18.7um. The combined limits
from MIDI and 8m imaging constrain the warm dust to lie within 0.05-2.4AU for
HD69830 and 0.16-2.98AU for eta Corvi. These results represent the first
resolution in the mid-IR of dust around main sequence stars. The constraints
placed on the location of the dust are consistent with radii predicted by SED
modelling. Tentative evidence for a common position angle for the dust at 1.7AU
with that at 150AU around eta Corvi, which might be expected if the hot dust is
fed from the outer disc, demonstrates the potential of this technique for
constraining the origin of the dust and more generally for the study of dust in
the terrestrial regions of main sequence stars.
