We report the discovery in TESS Sectors 3 and 4 of a compact triply eclipsing triple star system .
TIC 209409435 is a previously unknown eclipsing binary with a period of 5.717 days , and the presence of a third star in an outer eccentric orbit of 121.872 day period was found from two sets of third-body eclipses and from eclipse timing variations .
The latter exhibit signatures of strong 3rd-body perturbations .
After the discovery , we obtained follow-up ground-based photometric observations of several binary eclipses as well as another of the third-body eclipses .
We carried out comprehensive analyses , including the simultaneous photodynamical modelling of TESS and ground-based lightcurves ( including both archival WASP data , and our own follow-up measurements ) , as well as eclipse timing variation curves .
Also , we have included in the simultaneous fits multiple star spectral energy distribution data and theoretical PARSEC stellar isochrones .
We find that the inner binary consists of near twin stars of mass 0.90 M _ { \odot } and radius 0.88 R _ { \odot } .
The third star is just 9 % more massive and 18 % larger in radius .
The inner binary has a rather small eccentricity while the outer orbit has e = 0.40 .
The inner binary and outer orbit have inclination angles within 0.1 ^ { \circ } and 0.2 ^ { \circ } of 90 ^ { \circ } , respectively .
The mutual inclination angle is \lesssim 1 / 4 ^ { \circ } .
All of these results were obtained without radial velocity observations .