> orl.nl1 <- nls(ffe ~ b0 + ((b1*orl_dose)/(b2+orl_dose)), 
+ start=c(b0=1, b1=1, b2=1))
> 
> summary(orl.nl1)

Formula: ffe ~ b0 + ((b1 * orl_dose)/(b2 + orl_dose))

Parameters:
   Estimate Std. Error t value Pr(>|t|)    
b0    6.115      1.077   5.679 6.22e-08 ***
b1   27.625      3.476   7.948 3.21e-13 ***
b2  124.732     47.788   2.610  0.00991 ** 
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 

Residual standard error: 8.994 on 160 degrees of freedom

Number of iterations to convergence: 13 
Achieved convergence tolerance: 7.296e-06 

> vcov(orl.nl1)
           b0          b1         b2
b0  1.1594479  -0.7218709   15.60886
b1 -0.7218709  12.0809731  130.13601
b2 15.6088629 130.1360073 2283.72973
>