We use high-resolution Hubble Space Telescope imaging observations of the young ( \sim 15 - 25 Myr-old ) star cluster NGC 1818 in the Large Magellanic Cloud to derive an estimate for the binary fraction of F stars ( 1.3 < M _ { \star } / M _ { \odot } < 1.6 ) .
This study provides the strongest constraints yet on the binary fraction in a young star cluster in a low-metallicity environment ( \mbox { [ Fe / H ] } \sim - 0.4 dex ) .
Employing artificial-star tests , we develop a simple method that can efficiently measure the probabilities of stellar blends and superpositions from the observed stellar catalog .
We create synthetic color-magnitude diagrams matching the fundamental parameters of NGC 1818 , with different binary fractions and mass-ratio distributions .
We find that this method is sensitive to binaries with mass ratios , q \gtrsim 0.4 .
For binaries with F-star primaries and mass ratios q > 0.4 , the binary fraction is \sim 0.35 .
This suggests a total binary fraction for F stars of 0.55 to unity , depending on assumptions about the form of the mass-ratio distribution at low q .