Plotting circles on different planes

I need to plot circles in the zy and zx planes, but I can only plot in the xy plane, because by changing the variables x, y and z, I forget the shape of the circle. How can I fix this?

import numpy as np
from matplotlib import pyplot as plt
import mpl_toolkits.mplot3d.axes3d

theta = np.linspace(0, 2.*np.pi, 100)
phi = np.linspace(0, 2*np.pi, 100)
theta, phi = np.meshgrid(theta, phi)
c, a = 1, 0.05
x = (c + a*np.cos(theta)) * np.cos(phi)
y = (c + a*np.cos(theta)) * np.sin(phi)
z = a * np.sin(theta)

fig = plt.figure()
ax1 = fig.add_subplot(111, projection='3d')
ax1.set_zlim(-10,10)
ax1.plot_surface(x, y, z, color='b', edgecolors='b')
ax1.view_init(36, 26)

plt.show()