We have re-observed and re-analysed the optical spectrum of the Schweizer - Middleditch star , a hot subdwarf which lies along almost the same line-of-sight as the centre of the historic SN1006 supernova remnant ( SNR ) .
Although this object is itself unlikely to be the remnant of the star which exploded in 1006AD , Wellstein et al .
( 1999 ) have demonstrated that it could be the remnant of the donor star in a pre-supernova Type Ia interacting binary , if it possesses an unusually low mass .
We show that , if it had a mass of 0.1 - 0.2 \thinspace M _ { \odot } , the SM star would lie at the same distance ( \approx 800pc ) as the SNR as estimated by Willingale et al .
( 1995 ) .
However , most distance estimates to SN1006 are much larger than this , and there are other convincing arguments to suggest that the SM star lies behind this SNR .
Assuming the canonical subdwarf mass of 0.5 \thinspace M _ { \odot } , we constrain the distance of the SM star as 1050 pc < d < 2100 pc .
This places the upper limit on the distance of SN1006 at 2.1 kpc .