I present a new analysis of the MACHO Project 5.7 year Large Magellanic Cloud ( LMC ) microlensing data set that incorporates the effects of contamination of the microlensing event sample by variable stars .
Photometric monitoring of MACHO LMC microlensing event candidates by the EROS and OGLE groups has revealed that one of these events is likely to be a variable star , while additional data has confirmed that many of the other events are very likely to be microlensing .
This additional data on the nature of the MACHO microlensing candidates is incorporated into a simple likelihood analysis to derive a probability distribution for the number of MACHO microlens candidates that are true microlensing events .
This analysis shows that 10-12 of the 13 events that passed the MACHO selection criteria are likely to be microlensing events , with the other 1-3 being variable stars .
This likelihood analysis is also used to show that the main conclusions of the MACHO LMC analysis are unchanged by the variable star contamination .
The microlensing optical depth toward the LMC is \tau = 1.0 \pm 0.3 \times 10 ^ { -7 } .
If this is due to microlensing by known stellar populations , plus an additional population of lens objects in the Galactic halo , then the new halo population would account for 16 % of the mass of a standard Galactic halo .
The MACHO detection exceeds the expected background of 2 events expected from ordinary stars in standard models of the Milky Way and LMC at the 99.98 % confidence level .
The background prediction is increased to 3 events if maximal disk models are assumed for both the Milky Way and LMC , but this model fails to account for the full signal seen by MACHO at the 99.8 % confidence level .