N=2 Instanton Effective Action in Omega-background and D3/D(-1)-brane System in R-R Background

We study the relation between the ADHM construction of instantons in the Omega-background and the fractional D3/D(-1)-branes at the orbifold singularity of C \times C^2/Z_2 in Ramond-Ramond (R-R) 3-form field strength background. We calculate disk amplitudes of open strings connecting the D3/D(-1)-branes in certain R-R background to obtain the D(-1)-brane effective action deformed by the R-R background. We show that the deformed D(-1)-brane effective action agrees with the instanton effective action in the Omega-background.