Surface photometry, mass distributions and stability of the NGC 3686 quartet of galaxies

Surface photometry and long slit spectroscopy is presented for the NGC 3681/84/86/91 quartet of galaxies. Total B magnitudes are 11.81, 11.92, 11.59 and 12.41 respectively after correction for galactic absorption. Total mass estimates based on partial rotation curves for NGC 3684/86/91 are 4.9, 3.2 and 2.8 respectively in units of $${10}^{10}\,{M}_{\odot}$$. Rigorous formulae are given for the uncertainties in the energy of the system which is $$4.58\,\pm\,5.42\,\times\,{10}^{57}\,\text{erg}$$. The crossing time for the quartet is $$0.8\,\times\,{10}^{9}\,\text{yr}$$. The estimated crossing time for the G49 group, of which the quartet form a prominent subgroup, is $$9\,\times\,{10}^{9}\text{yr}$$.