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cluster in projection. Given the core radius derived by Mathieu (1983) of 1.9 ± 0.1 pc, our sample is drawn from the inner ∼ 4 core radii. 

Note that we lack BV photometry for ∼ 11% of the point sources found within 30 arcminutes from the clus- ter center in 2MASS. For most of these objects, it is likely that there is a nearby or overlapping additional object which has prevented accurate BV photometric measure- ments from either of our sources. These objects, by de- fault, are not included in our stellar sample. In total, our stellar sample contains 1344 stars. 

Since September 1997, we have collected 5201 spec- tra of 1144 stars within this stellar sample as part of an ongoing observing program using the WIYN Hydra MOS. For the majority of these observations, we use Hy- dra’s blue-sensitive 300 µ m fibers, which project to a 3.1 ′ aperture on the sky. We use the 316 lines mm − 1 echelle grating, isolating the 11th order with the X14 filter. The resulting spectra span a wavelength range of ∼ 25 nm, with a dispersion of 0.015 nm pixel − $^{1}$, centered on 512.5 nm. We have also occasionally centered our observation on 637.5 nm using a very similar setup. In this region, we use the same grating, but isolate the 9th order with the X18 filter. These observations span a slightly larger wavelength range of ∼ 30 nm, and have a dispersion of 0.017 nm pixel − $^{1}$. Due to a broken filter, observations taken after the spring of 2008 use different observing se- tups than discussed above; most are centered on 560 nm and all use the echelle grating. We have not noticed any decrease in performance from the new wavelength range, but we caution the reader that we lack sufficient obser- vations in these setups to reliably determine our RV pre- cision for these measurements. During this same period certain upgrades were made to the spectrograph collima- tor $^{5}$. All observed regions are rich in metal lines. The typical velocity resolution is 15 km s − $^{1}$. In a two-hour integration, the spectra have signal-to-noise (S/N) ra- tios ranging from ∼ 18 per resolution element for V =16.5 stars to ∼ 100 per resolution element for V =13 stars. 

We create fiber configurations ( pointings ) for our ob- servations using a similar method as Geller et al. (2008). Monte Carlo simulations show that we require at least 

## 2.2. Spectroscopic Observations 

three observations over the course of a year in order to en- sure 90% confidence that a star is either constant or vari- able in RV out to binary periods of 1000 days (Mathieu 1983, Geller & Mathieu, in preparation). Given three observations with consistent RV measurements over a timespan of at least a year and typically longer, we clas- sify a given star as single (strictly, non-RV variable) and finished, and move it to the lowest priority. If a given star has three RV measurements with a standard deviation > 2.0 km s − 1 (four times our precision for narrow-lined stars; see Section 4.2), we classify the star as RV vari- able and give it the highest priority for observation on a schedule appropriate to its timescale of variability. This prioritization allows us to most efficiently derive orbital solutions for our detected binaries. 

We place our shortest-period binaries at the highest priority for observations each night, followed by longer- period binaries to obtain 1-2 observations per run. Be- low the confirmed binaries we place, in the following or- der, “candidate binaries” (once-observed stars with a RV measurement outside the cluster RV distribution or stars with a few measurements that span only 1.5 - 2.5 km s − $^{1}$), once observed and then twice observed non-RV- variable likely members, twice observed non-RV-variable likely non-members, unobserved stars, and finally, “fin- ished” stars. Within each group, we prioritize by dis- tance from the cluster center, giving those stars nearest to the center the highest priority. A typical pointing will contain ∼ 70 fibers placed on individual stars in our sample and ∼ 10 sky fibers. 

For a given pointing we obtain three consecutive ex- posures, each of 40 minutes. In poor transparency or with a particularly bright sky, we restrict the targets to V < 15 . 0 and shorten the integration time, gener- ally to 20 minute exposures. We obtain Thorium-Argon (ThAr), or occasionally Copper-Argon (CuAr), emission- lamp comparison spectra (300 s integrations) before and after each set of science integrations for wavelength cal- ibration and to check for wavelength shifts during the observing sequence. For each set of integrations we also obtain one flat-field image (200 s) of a white spot on the dome illuminated by incandescent lights. Associat- ing the flat-field images with the science integrations is particularly critical for calibrating throughput variations between the fibers in order to apply sky subtractions. In total, we have observed 106 distinct pointings in M35 

