Create complex legend with two string including variables values.

I'm trying to add a legend to a plot where multiple variable are plotted in two y axes. I want to highlight two values on this plot (concerning only one axis) and add a legend about only this points.

here is the code:

p5=plot(U(kk1),real(sol(kk1,:)),'d','MarkerSize',15,'Color','b');
p6=plot(U(kk2),real(sol(kk2,:)),'o','MarkerSize',15,'Color','b');

str_lgd1 =['Real eigenvalue for U = ',...
    num2str(U_stop(1)), ' m/s' char(10) 'Re(p) = ', ...
    num2str(abs(real(sol(kk1,1))))];
str_lgd2 = ['Real eigenvalue for U = ',...
    num2str(U_stop(2)), ' m/s' char(10) 'Re(p) = ', ...
    num2str(abs(real(sol(kk2,1))))];

lgd=legend([p5 p6],str_lgd1,str_lgd2);

p5 and p6 are the objects I want to add the legend. str_lgd1 and str_lgd2 are the string I want to be displayed. The error is:

Operands to the || and && operators must be convertible to logical scalar values.

Error in legend (line 189)
        all(isgraphics(args{1})) % legend(children,strings,...)

Error in Es4 (line 244)
lgd=legend([p5 p6],{str_lgd1,str_lgd2});