High-mass binaries - a unique laboratory to study the formation and evolution of stars and for measuring cosmic distances

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High-mass stars seem to have the highest multiplicity rate among all stars which increases the chance to be observed also as eclipsing binary (EB) systems. As a consequence early-type EBs are not only a unique laboratory to study properties, of high-mass stars but – due to their brightness – they have also the potential for measuring accurate distances to nearby galaxies, and thus to significantly improve the Hubble constant determination based on Cepheids and SN Ia.Our archive, containing about 3000 O- and 5000 B-star spectra, represents a solid – probably the largest – collection of homogeneous, multi-epoch, high S/N spectra for high-mass stars – among them about 80 SB2 systems in the range O3 – O9 and 90 SB2 systems in the range B0 – B9. Likewise, our multiband photometric Galactic Disk Survey of the southern Milky Way has produced high-precision light curves over a period of almost 10 years and revealed about 85,000 variable objects of which more than 85% were not known previously as variables. This photometric archive comprises 473 O and more than 1000 B stars – among them 41 O-type EBs and about 300 B-type EBs.With the present project we want to go into a detailed investigation of interesting objects by combining our high-quality EB light curves with our multi-epoch high-resolution spectra to attack the major questions with respect to high-mass stars, i.e.- Stellar parameters like mass, mass-ratio, luminosity, radius, abundances, period, etc.- Origin and evolution of close multiple and interacting systems - Detection and investigation of higher multiplicity systems- Mass transfer and rotation of individual components in multiple systemsApart from exploring stellar properties, detached double-lined spectroscopic EBs offer a unique opportunity to measure accurate distances to nearby galaxies. By high-precision V – K measurements of early-type EBs we will solve the main problem related to the calibration of the surface brightness – color relation technique making it an excellent tool for obtaining 2% distances to individual EB systems in a volume of 2 Mpc now, and up to 30 Mpc with the advent of future extremely large telescopes.During the present project we will measure 1.5 – 2% geometrical distances to two systems in M31, one system in M33 and one in IC 1613 and thus will significantly improve distance determinations to these galaxies based on the EBs.In particular we will calibrate the dependence of P-L relations of Cepheids on metallicity which will be a fundamental and necessary step towards a Hubble constant determination accurate to 1%.The combination of different expertizes of both teams will allow to exploit huge potential of binary systems to study both star formation and evolution, and improve the calibration of the extragalactic distance scale. The obtained results will have strong impact on many different fields of modern astrophysics: star formation, stellar evolution, studies of nearby galaxies and cosmology.