1 Introduction

Low-mass stars constitute the most numerous stellar population of the Galaxy. Yet, their physical properties are still poorly known. This is not without the efforts of many investigators that have been working intensively on the modelling of the atmosphere and interior structure of sub-solar mass stars. The major problem that the field has to face is a conspicuous lack of suitable calibrators. Unfortunately, very few main-sequence M stars have empirically-determined masses, radii, luminosities and temperatures.

Eclipsing binaries have often served as valuable benchmarks for the validation of structure and evolution models (see, e.g., Andersen 1991). Despite some unsolved problems at the high-mass end of the main sequence, the behaviour of stars more massive than the Sun is well understood, partly thanks to the large number of eclipsing binaries with well-determined physical properties. The situation is radically different at the low-mass end of the main sequence. Selection effects caused by the faintness of the stars and the often strong intrinsic variations due to magnetic activity pose a serious challenge for the discovery and analysis of detached eclipsing binaries. Only three detached eclipsing systems with dwarf M-type components have been identified to date. The member of the Castor multiple system YY Gem has almost identical components of spectral type M1 Ve and mass $\sim$0.6 $M_{\odot }$ (Torres & Ribas 2002, hereafter TR02). CM Dra holds the record for the lowest mass eclipsing binary known and has components of $\sim$0.25 $M_{\odot }$ and spectral type M4.5 Ve (Metcalfe et al. 1996). These were the only two detached M-type eclipsing binaries known for decades until Delfosse et al. (1999a, hereafter D99) recently reported the discovery of eclipses in CU Cnc.

CU Cnc (GJ 2069A, HIP 41824, $\alpha=8^{\rm h} 31^{\rm m} 37\hbox{$.\!\!^{\rm s}$ }58$, $\delta=+19\hbox{$^\circ$ } 23\hbox{$^\prime$ }39\hbox{$.\!\!^{\prime\prime}$ }5$) is an 11.9-mag spectroscopic binary with M3.5 Ve components. Also, it has a fainter visual companion, CV Cnc or GJ 2069B, at an angular distance of $\sim$12 $^{\prime\prime}$. GJ 2069B is itself a close binary star (Delfosse et al. 1999b) with a mean spectral type of M4 and appears to form a physically bound quadruple system with CU Cnc. Interestingly, the masses found by D99 place CU Cnc nicely in between of YY Gem and CM Dra and make it a system of paramount importance to improve upon our knowledge of the physics of low-mass stars.

Currently, the high-precision spectroscopic orbit of CU Cnc presented by D99 (errors in minimum mass below 0.4%) is not matched by the photometric light curves, and the authors reported only sparse photoelectric observations. The motivation of our study was to acquire and analyse new high-quality photometric observations with the aim of determining accurate masses and radii for the components of CU Cnc. With a careful estimation of the temperature, our goal is to compare the physical properties of CU Cnc's components with the predictions of the available low-mass stellar evolution models.