### Accuracy of orbital period measurement

Let’s say you have measured the $HJD_0$ at times $t_1$ and $t_2$, separated by N orbital cycles. Then the orbital period, $P$, becomes: $P=\dfrac{t_{2} - t_{1}}{N}$…(1)

From error propagation theory we know that if: $P=f(t_1,t_2,N)$

then: $\sigma^2_P=\left(\dfrac{\partial P}{\partial t_{1}} \sigma_{t_{1}}\right)^2+\left(\dfrac{\partial P}{\partial t_{2}} \sigma_{t_{2}}\right)^2$

in which we are ignoring the partial derivative of $P$ with respect to $N$ because we consider $\sigma_N$ to be zero. The equation becomes: $\sigma^2_P=\left(\dfrac{-\sigma_{t_1}}{N}\right)^2+ \left(\dfrac{\sigma_{t_2}}{N}\right)^2=\dfrac{\sigma_{t_1}^2+ \sigma_{t_2}^2}{N^2}$

and thus: $\sigma_P=\dfrac{1}{N}\sqrt{\sigma_{t_1}^2+ \sigma_{t_2}^2}$…(2)

Then using (1) in (2): $\sigma_P=\dfrac{P}{t_2 - t_1}\sqrt{\sigma_{t_1}^2+ \sigma_{t_2}^2}$…(3)

In real life $t_1$ and $t_2$ are typically reported with uncertainties of the order $1 \times 10^{-4}$ d. Therefore, for a system with period $\sim6$ h and $t_1$, $t_2$ separated by one year we expect an uncertainty of $\sim 1 \times 10^{-7}$ d for the orbital period.

### Roche lobe plots

For all the following I’m using gnuplot.

Isopotential curves a=1
q=0.4
f(x,y)=2/(((x**2+y**2)**2)*(1+q)) +2*q/((((x-a)**2+y**2)**2)*(1+q)) +(x – q/(1+q))**2 +y**2
set view 0,0,1.3
set xlabel “x”
set ylabel “y”
set contours
set isosamples 100,100
set size ratio -1
unset surface
set cntrparam levels disc -2.2,-2.5,-3.55,-29.7,-100,-1000
unset ztics
splot [x=-6:6] [y=-6:6] -f(x,y)