Ok, well, according to the H-R diagramme (placing the cluster's H-R diagramme in the Milky Way's), the brightness of the stars like Alcyone, Merope, Atlas, Electra, Maia from M45, show they're 423 light-years away.
But in 1997, Hypparcos parallax measures located M45 at only 378 light-years.....Problem....Who's right? (45 light-years difference, far too much).
So, a third way to measure distances is being thought.
Atlas has a stellar companion around it apparently! Ok, why no then add to astrometric observations, spectroscopic measurements of the radial speed and deduce the real size of the second star's orbit? Comparing it with the apparent size, known through interferometry, astronomers would get Atlas' distance. But one can wonder if Atlas really is the center of M45. The star is massive and is a binary and can only move very slowly from the center, according to many.
Measurements of the radial velocity of very massive stars are blurred by their rapid rotation. That's why data from previous measurement are needed, for the estimation.
At least 290 days will be needed for the measurements (that's one orbit of Atlas' companion) and the OHP in France will be used for this.
So, do we know enough about chemical composition of stars or is there a systematic mistake in Hypparcos parallaxes?
We'll find out.

Raoul