We present TYC 2505-672-1 as a newly discovered and remarkable eclipsing system comprising an M-type red giant that undergoes a \sim 3.45 year long , near-total eclipse ( depth of \sim 4.5 mag ) with a very long period of \sim 69.1 yr. TYC 2505-672-1 is now the longest-period eclipsing binary system yet discovered , more than twice as long as that of the currently longest-period system , \epsilon Aurigae .
We show from analysis of the light curve including both our own data and historical data spanning more than 120 yr and from modeling of the spectral energy distribution , both before and during eclipse , that the red giant primary is orbited by a moderately hot source ( T _ { eff } \approx 8000 K ) that is itself surrounded by an extended , opaque circumstellar disk .
From the measured ratio of luminosities , the radius of the hot companion must be in the range 0.1–0.5 R _ { \odot } ( depending on the assumed radius of the red giant primary ) , which is an order of magnitude smaller than that for a main sequence A star and 1–2 orders of magnitude larger than that for a white dwarf .
The companion is therefore most likely a “ stripped red giant ” subdwarf-B type star destined to become a He white dwarf .
It is however somewhat cooler than most sdB stars , implying a very low mass for this “ pre-He-WD ” star .
The opaque disk surrounding this hot source may be a remnant of the stripping of its former hydrogen envelope .
However , it is puzzling how this object became stripped , given that it is at present so distant ( orbital semi-major axis of \sim 24 AU ) from the current red giant primary star .
Extrapolating from our calculated ephemeris , the next eclipse should begin in early UT 2080 April and end in mid UT 2083 September ( eclipse center UT 2081 December 24 ) .
In the meantime , radial velocity observations would establish the masses of the components , and high-cadence UV observations could potentially reveal oscillations of the hot companion that would further constrain its evolutionary status .
In any case , this system is poised to become an exemplar of a very rare class of systems , even more extreme in several respects than the well studied archetype \epsilon Aurigae .