It seems to me that the only reason why there
is a spectral type in xephem is to draw star
colors: blue for O,B,A, yellow for F,G,K
and red for M. Notice that there are only 3
different colors.
There is no spectral type in tycho2, but
there is the Johnson color index B-V, which
describes the color of the star, and from which
the spectral type can be derived.

I have found some numbers on the internet:
http://einstein.stcloudstate.edu/Dome/constellns/bv.html

And use them in my conversion utility:

const char *BvToSpectr(double b_v) {
static char sp[3];
if (b_v