First photometric study of W UMa binary star LU Lac

•The first BVRI CCD photometric light curves were presented.•The first photometric solutions and orbital period analyses for LU Lac.•The photometric solutions suggest LU Lac is a marginal contact W-type system.•Orbital period analyses suggest it may be a triple system. LU Lac is a neglected W UMa binary star in photometric investigations. In this paper, we present BVRI CCD photometric light curves obtained on one night in 2012. The first photometric solutions of this system are computed by using the Wilson–Devinney code. It is shown that LU Lac is a marginal contact W-type system with a degree of contact factor of f=8.9%, a mass ratio of q=2.085 and a high inclination of i=82°.20. From the first analyses of orbital period changes, we found the period variation of the system includes an oscillation (A3=0.0125 days and T3=51.92 years). The cyclic change may be attributed to the light-travel time effect through the presence of a third body.