Lumens Corp is trying to determine its optimal capital structure. The companys capital structure consists of debt and common stock. To estimate the cost of debt, the company has produced the following table:

Percent financed with debt (wd)	Percent financed with equity (wc)	Debt-to-equity ratio (D/S)	After-tax cost of debt (%)
0.25	0.75	0.25/0.75 = 0.33	6.5%
0.35	0.65	0.35/0.65 = 0.5385	7.4%
0.50	0.50	0.50/0.50 = 1.00	8.0%

The company uses the CAPM to estimate its cost of common equity, r_s. The risk-free rate is 5% and the market risk premium is 6%. Lumens estimates that its beta with 10% debt is 1. The companys tax rate, T, is 40%. Based on this information, what is the companys optimal capital structure, and what is the firms cost of capital at this optimal capital structure?