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We present Keck I/OSIRIS and Keck II/NIRC2 adaptive optics imaging of twomember candidates of the Praesepe stellar cluster (d=186.18$\pm$0.11 pc;590-790 Myr), UGC J08451066+2148171 (L1.5$\pm$0.5) and UGCS J08301935$+$2003293(no spectroscopic classification). We resolved UGCS J08451066$+$2148171 into abinary system in the near-infrared, with a $K$-band wavelength flux ratio of0.89$\pm$0.04, a projected separation of 60.3$\pm$1.3 mas (11.2$\pm$0.7 au;1$\sigma$). We also resolved UGCS J08301935$+$2003293 into a binary system witha flux ratio of 0.46$\pm$0.03 and a separation of 62.5$\pm$0.9 mas. Assumingzero eccentricity, we estimate minimum orbital periods of $\sim$100 years forboth systems. According to theoretical evolutionary models, we derive masses inthe range of 0.074-0.078 M$_{\odot}$ and 0.072-0.076 M$_{\odot}$ for theprimary and secondary of UGCS J08451066$+$2148171 for an age of 700$\pm$100Myr. In the case of UGCS J08301935$+$2003293, the primary is a low-mass star atthe stellar/substellar boundary (0.070-0.078 M$_{\odot}$) while the companioncandidate might be a brown dwarf (0.051-0.065 M$_{\odot}$). These are the firsttwo binaries composed of L dwarfs in Praesepe. They are benchmark systems toderive the location of the substellar limit at the age and metallicity ofPraesepe, determine the age of the cluster based on the lithium depletionboundary test, derive dynamical masses, and improve low-mass stellar andsubstellar evolutionary models at a well-known age and metallicity.

Two close binaries across the hydrogen-burning limit in the Praesepe open cluster