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We have obtained new photometry and intermediate resolution ($\Delta \lambda= 2.7$ \AA\ ) spectra of 19 of these objects  (14.9 $\le$ $I_c$ $\le$ 17.5) in order to confirm cluster membership. Weidentify 15 of our targets as likely cluster members based on their $VRI$photometry, spectral types, radial velocity, and H$\alpha$ emission strengths.Higher S/N spectra were obtained for 8 of these probable cluster members inorder to measure the strength of the lithium 6708 \AA\ doublet and thus obtainan estimate of the cluster's age. One of these 8 stars has a definite lithiumdetection and two other (fainter) stars have possible lithium detections. Acolor-magnitude diagram for our program objects shows that the lithiumdepletion boundary in IC~2391 is at $I_c$=16.2. Using recent theoretical modelpredictions, we derive an age for IC~2391 of 53$\pm$5 Myr. While this isconsiderably older than the age most commonly attributed for this cluster($\sim$35 Myr) this result for IC~2391 is comparable those recently derived forthe Pleiades and Alpha Persei clusters and can be explained by new models forhigh mass stars that incorporate a modest amount of convective coreovershooting.Comment: ApJ Letters, acccepte

The Lithium-Depletion Boundary and the Age of the Young Open Cluster IC 2391