# IV regression: endogenous variable is sum of exogenous and endogenous

Suppose I construct the variable $U_i = S_i + T_i$, and I want to estimate the equation

$$Y_i = \alpha + \beta U_i + \gamma X_i + \epsilon _i$$

Suppose $X_i$ and $S_i$ are exogenous, but $T_i$ is endogenous.

A valid instrument for $X_i$ appears to be be $S_i$, so that the first stage estimation would be

$$U_i = \alpha_U + \beta_U S_i + \gamma_U X_i + \eta_i$$

Since $S_i$ is used in the construction of $U_i$, it should be a relevant instrument, and since we assume that $S_i$ is exogenous, it should also satisfy the exclusion restriction.

Supposing that the above is correct, my question is whether the IV estimation of $\beta$ is the same as just estimating $\beta_S$ in the following equation:

$$Y_i = \alpha_S + \beta_S S_i + \gamma_S X_i + \xi_i$$