The MONS project is equipped with three different cameras that will acquire accurate photometry of around 6000 stars through a period of two years. The two star trackers, with 22-degree field of views and pointing 180 degrees apart, will be very useful instruments for obtaining light curves of known eclipsing binaries and discovering new ones.

A number of papers have dealt with the MONS contributions to the study of eclipsing binaries. See, for example, the proceedings of the First and Third MONS Workshops. Particular aspects of eclipsing binaries are studied within the context of MONS (eccentric systems, systems with pulsating components, second order light curve effects). In contrast, this web page is intended to be a short general summary of the wide range of astrophysical problems that can be studied with eclipsing binaries. We also present some numerical estimates and examples of what contributions can be expected from the MONS project.

What are eclipsing binaries useful for?

The most interesting fact about eclipsing binaries is that all kinds of stars are found as members of binaries: from normal main sequence stars, variable stars, evolved giants and supergiants, to collapsed objects. In addition to making excellent astrophysical laboratories for the study of the common interaction between stars (mass transfer, accretion processes, tidal evolution, binary star evolution, etc), they also yield fundamental physical properties of the components though the analysis of light and radial velocity curves. The following table summarizes the most remarkable properties of each major class of eclipsing binaries and their importance to Astrophysics:

Detached Eclipsing Binaries Masses, radii and luminosities. Provide tests of stellar structure and evolution models. Mass-Luminosity relationship. Distances to clusters and galaxies.
Eclipsing Binaries in Eccentric Orbits Masses, radii, luminosities, and internal structure of stars. Further checks on stellar models, tidal evolution models and tests of General Relativity.
Eclipsing Binaries with Variable Components Studies of d Scu variables, Cepheids, b CMa variables, slowly-pulsating B stars, etc.
Chromospherically Active Binaries (RS CVn, BY Dra) Magnetic activity, star spots, chromospheric and coronal emissions, "solar-stellar" connection.
Semi-detached Binaries (Algol-type, W Ser) Binary star evolution, mass loss and mass exchange, accretion processes, ISM enrichment.
Contact Binaries (cool systems: W UMa) Stellar activity and magnetism, binary star evolution and coalescence, angular momentum loss.
Contact Binaries (hot systems: AO Cas, WR binaries) Binary star evolution and dynamics, mass loss, interacting winds.
Near-contact Binaries (V1010 Oph) Stellar evolution, mass loss and mass transfer, magnetic activity in systems with cool components.
z Aur and VV Cep systems Masses, radii and structure of evolved supergiant stars, mass loss, tests of evolution models.
Binaries with degenerate components (CVs, NLVs, XRBs, Symbiotics, Post-common envelope) Properties of degenerate objects, binary star evolution, accretion processes and accretion disks, atmospheric eclipses.

Adapted from Guinan (1993, ASP Conf. Series, Vol. 38, p. 1)

Eclipsing Binaries with MONS

The main asset of MONS with respect to eclipsing binaries is its capability of obtaining stable, long time series photometry of moderately bright stars with millimagnitude-level accuracy. The extended time baseline will guarantee an excellent phase coverage of systems with short and intermediate periods (up to a few tens of days). Here is an illustrated example:

  This is the light curve of a 4-day period detached eccentric eclipsing binary. The error of an individual error is around 0.01 mag, which is about what the FST of MONS will be capable of for 7th magnitude stars. This plot also illustrates the density in the phase coverage that will be achieved. Actually, even better coverage than shown here is expected.

Let us see now a more specific example, namely the evolved eclipsing binary V380 Cyg. This is a system composed of an evolved massive giant star with a less massive companion still in the main sequence. This system has proved to be an extremely useful astrophysical laboratory, but its long period (about 12 days), and its shallow and long-lasting eclipses make light curve observations difficult. The plot below shows one of the best light curves ever secured for V380 Cyg:

  This is a yellow light curve of the 5.5-mag eclipsing binary V380 Cyg. The individual error of one measurement is about 6 millimagnitudes. The scatter seems apparently larger, though, because the eclipses are only 0.1 magnitudes deep. Note that the phase coverage is not very good, especially along the eclipse branches. The FST of MONS will be able to achieve far better phase coverage with a very similar photometric accuracy. This will dramatically improve the quality of the derived photometric elements, including the apsidal motion rate.

We have picked the particular example of V380 Cyg for a reason. Hegedüs and Nuspl carried out a study of the stellar fields covered by the star trackers while MONS is observing the five primary targets. V380 Cyg is a good example because it will be indeed well-suited for observation by the FST when the primary target HR3018 is being monitored.


The examples shown above demonstrate that MONS can yield outstanding results. A few particular items in which MONS will contribute significantly by observing light curves of known eclipsing binaries are:

In addition to observing well-selected known eclipsing binaries, MONS also has the potential to discover a significant number of new systems. Statistical estimates based upon large scale surveys indicate that over 1% of the monitored stars will turn out to be eclipsing binaries. As discussed in these pages, the outcome of the MONS project will certainly be of great benefit to the eclipsing binary community.

Ignasi Ribas (